Thermostatic mixing valves are known and their use is greatly spreading in developed countries mainly due to regulations enforcing, at least one central thermostatic mixing valve, in new installations. However, cost reduction, as well as recognition of the convenience and safety involved, contributes to increasing market share of thermostatic mixing valves in point of use installations such as individual basins, bathtubs and showers, as well.
Presently, most thermostatic mixing valves utilize a thermally responsive wax element directly coupled to a spring loaded proportioning valve. However, such directly driven thermostatic valves fail to provide a constant outlet water temperature, if the pressure or the temperature of the water in one of the supply lines, rises or drops, the temperature of the outlet water is temporarily changed. The temperature responsive wax element responds to the temperature change by forcing the proportioning valve in the direction that will tend to restore the mixed water temperature to its previous level, nevertheless, the previous level can not be reached as long as the supply line conditions are altered, and the system will settle in a new equilibrium position corresponding to a new outlet water temperature. This new outlet water temperature is not the preselected temperature since, due to its linear characteristic, any new position of the temperature responsive wax element, different from the initial position, is associated with a different mixed water temperature.
Consequently, a typical error of up to 2° C. from the initial setting, can be expected in wax element thermostatic valves, in the event of pressure or temperature fluctuations in one of the supply lines. Furthermore the response time of such wax filled elements is slow, typical temperature restore times after pressure fluctuation events are in the range of 1.5 seconds or above, such a delay may cause inconvenience to the user, as well as oscillations or hunting mostly apparent at low flow rates.
Another, in theory, more precise approach to control thermostatic mixing valves is to employ an electric amplified feedback device. Electric operated thermostatic mixing valves usually comprises hot and cold water inlets, a motor with speed reducing gear or lead screw, driving rotational or linear proportioning valve, a mixing chamber, a temperature sensor, an electronic comparator unit for comparing the temperature sensor reading with a reference signal, and a motor controller for keeping the signal differences as low as possible.
Examples of such electric powered thermostatic mixing valves can be found for instance in U.S. Pat. Nos. 4,359,186 4,420,811 4,931,938 5,944,255 and U.K. patent GB2056627A.
The described systems typically include sophisticated electronics, a microprocessor running some complex mathematical model, an amplifier stage suitable for driving at least 10 W motor, further requiring high output electrical power supply. Safety protection against electrical shock is needed, as well as protection against power loss to avoid the risk of losing control of the mixed water temperature.
In an attempt to detect the basic reasons for such complexity, two main deficiencies of the prior art were isolated:
a. Most of the disclosed inventions are using conventional proportioning valves fitted with an electric drive unit. These devices require significant force to overcome pressure imbalances, friction of a fluid-to-air seal and friction of internal proportioning valve fluid-to-fluid seals. This in turn requires large motors having high power consumption and high mechanical inertia, further requiring computing of dedicated acceleration and deceleration algorithms, large power supply, and in-wall installation extending to AC mains power.
b. Additionally, the conventional proportioning valves generally have large volume mixing chamber resulting in a delayed reading of the mixed water temperature by the temperature sensor, such delay makes it very difficult to effectively control the proportioning valve in different flow rates, since the time required for the hot and cold water to flow from the proportioning valve seats to the temperature sensor is considerably greater at low flow rates than during high flow rates, resulting in a too wide dispersion of the feedback system time constant. Some of the cited prior art are using two independent proportioning valves, having long pathways to the mixing chamber, and even longer time delay as described above.
A microprocessor loaded with complex computational model is aimed to overcome the time constant problem, by adjusting the steady state water temperature slowly and creating a high speed or momentary loop in case of disturbances in inlet water pressure or temperature, forming two distinct bands of operation, being the high speed or disturbance band and the set point or steady state band. However such solution requires additional sensors as well as computation resources requiring more power and space.
Consequently, although basic technology for electric operated mixing valves exists, a new approach for construction of the proportioning valve and its drive system is required.